This invention relates to systems and methods of reducing dark current levels in a standing wave linear accelerator.
Radiation therapy involves delivering a high, curative dose of radiation to a tumor, while minimizing the dose delivered to surrounding healthy tissues and adjacent healthy organs. Therapeutic radiation doses may be supplied by a standing wave linear accelerator that is configured to generate a high-energy (e.g., several MeV) electron beam. In an electron mode of operation, the electron beam may be applied directly to one or more therapy sites on a patient. Alternatively, in a radiation mode of operation, the electron beam may be used to generate a photon (e.g., X-ray) beam that may be applied to the patient. The shape of the radiation beam at the therapy site may be controlled by discrete collimators of various shapes and sizes or by multiple leaves (or finger projections) of a multi-leaf collimator that are positioned to block selected portions of the radiation beam. The multiple leaves may be programmed to contain the radiation beam within the boundaries of the therapy site and, thereby, prevent healthy tissues and organs that are located beyond the boundaries of the therapy site from being exposed to the radiation beam.
In general, a standing wave linear accelerator includes a particle source (e.g., an electron gun) that directs charged particles (e.g., electrons) into an accelerating cavity. The charged particles travel through a succession of accelerating cavities, where the particles are focused and accelerated by an electromagnetic (RF) field that is applied by an external RF source (e.g., a klystron or a magnetron). Additional electrons may be introduced into the standing wave linear accelerator by sources other than the cathode of the electron gun. These additional electrons are accelerated in the linear accelerator to produce an undesirable background xe2x80x9cdark currentxe2x80x9d. As used herein, xe2x80x9cdark currentxe2x80x9d refers to the electron beam current that is produced by an accelerator system when the electron gun is turned off and the RF source is turned on. Among the sources of dark current in standing wave linear accelerators is the electron gun itself. When the electron gun is hot (i.e., the filament is on and the gun is warmed up), the cathode will emit electrons in the presence of an accelerating voltage. When the gun is off, electrons also may be emitted from the grid structure that is used to bias the gun off. The first half cavity in the standing wave linear accelerator is another source of dark current. In particular, over time, this cavity may become coated with oxides that are produced by the gun cathode, especially when the gun is run above rated current levels. These oxide coatings reduce the work function of the cavity surfaces such that a low current electron beam may be produced in the presence of high electric fields, even if the cavity surfaces are cool.
Dark current introduces undesirable beam that reduces the ability to measure and control the therapeutic beams produced by standing wave linear accelerators. In addition, the background dark current beam may interfere with the associated imaging system and, consequently, may result in a poor or inaccurate diagnosis of a patient.
The invention features systems and methods for reducing dark current levels in a standing wave linear accelerator without sacrificing operating performance.
In one aspect, the invention features a method of generating a therapeutic beam. In a radiation mode, a standing wave linear accelerator is operated to produce a pulsed therapeutic photon beam having a characteristic pulse width. In an electron mode, the standing wave linear accelerator is operated to produce a pulsed therapeutic electron beam having a characteristic pulse width that is shorter than the characteristic pulse width of the therapeutic photon beam.
In some embodiments, assuming a uniform dark current level, the dark current level may be reduced in proportion with the beam pulse width reduction in the electron mode of operation.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The therapeutic photon beam and the therapeutic electron beam preferably have substantially the same pulse repetition rate.
The therapeutic photon beam may be produced by intercepting a pulsed electron source beam with an x-ray target. The electron source beam typically has a lower beam current than the therapeutic electron beam.
The pulse width of the therapeutic photon beam may correspond to a factory-preset pulse width. The pulse width of the therapeutic electron beam may be adjusted to reduce dark current produced by the standing wave linear accelerator. The pulse width of the therapeutic electron beam may be adjusted to substantially correspond to a characteristic fill time for the standing wave linear accelerator.
The therapeutic electron beam current level may be adjusted to accommodate adjustment of the pulse width of the therapeutic electron beam. The electron beam current level may be adjusted proportionately with the pulse width adjustment to maintain a desired dosage level.
In one embodiment, the therapeutic photon beam has an energy level of about 1 MeV or greater, and the therapeutic electron beam has an energy level of about 4-24 MeV.
In another aspect, the invention features a system for implementing the above-described therapeutic beam generation method.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.